2.3 Definition of LBBAP and LBBP
If the RBBB pattern in V1 was observed during unipolar pacing, it was considered a success for LBBAP regardless of the evidence of LBB capture. Therefore, the LBBAP includes deep septal pacing, nonselective LBB pacing (LBBP), and selective LBBP. To determine successful LBB capture, we followed and revised the definition of procedural success of LBB capture that was used in previous studies by Vijayaraman et al.12 and Wu et al.13 If the RBBB configuration was seen in V1 during the unipolar pacing in addition to one or more of the following findings, the success of LBBP was confirmed: 1) Abrupt shortening of Stim-LVAT (stimulus to peak of the R wave in V6) of >10ms during increasing output; 2) Short and constant stim-LVAT and the shortest stim-LVAT <75ms in non-LBBB and <85ms in LBBB; 3) Programmed stimulation by pacing lead changes QRS morphology from nonselective LBB to LV septal capture; 4) LBB potential (LBB-V interval of 15 to 35ms); and 5) Transition from nonselective LBB capture to selective LBB capture at near threshold outputs. If LBB capture could not be demonstrated, further advancement of the lead tip was guided by monitoring the unipolar pacing impedance and/or observation of the fixation beat. Examples of intracardiac electrograms obtained during the LBBAP procedure are presented in Supplementary Figure S3.